Half-bridge differential sensor

ABSTRACT

The present invention relates to a half-bridge signal processing circuit comprising a first and a second branch. The first branch comprises a first stimulus responsive sense element and a first current source arranged to provide a current to the first sense element. The second branch comprises a second stimulus responsive sense element and a second current source arranged to provide a current to said second sense element. The first and the second branch have a terminal in common. The first branch comprises a first node between said the current source and the first stimulus responsive sense element configured to generate a first signal related to a voltage over the first sense element. The second branch comprises a second node between the second current source and the second stimulus responsive sense element configured to generate a second signal related to a voltage over the second sense element.

FIELD OF THE INVENTION

The present invention is generally related to the field of half-bridgecircuits as used in sensors for monitoring an applied stimulus such aspressure, force, acceleration and the like.

BACKGROUND OF THE INVENTION

A sensor of the type considered in this invention is used, for example,in an automotive application for monitoring an applied stimulus, such aspressure (e.g. brake fluid pressure, exhaust gas pressure or cylinderpressure), force, acceleration and the like and, as in the case of apressure sensing application, typically comprises a sense element toprovide a signal in response to a target stimulus. A widely applicableelectrical representation of such a sensor (e.g. a pressure sensor) isthe half-bridge circuit shown in the picture FIG. 1.

The half-bridge circuit of FIG. 1 comprises two serially connected senseelements, represented by their equivalent resistances (Rpos, Rneg). In anominal condition of the physical magnitude being measured (e.g. zeropressure) and assuming an ideal sensor, these two serial connected senseelements have the same resistance value. When the physical magnitudechanges from that nominal condition, one of the resistances increasesproportionally (Rpos in FIG. 1) and the other (Rneg) decreasesproportionally. In the case of a pressure sensor the half-bridge circuitbehaviour vs the measured pressure can be described by the followingequations:Rpos=Rb*(½+((P/Pfs)*Sens)+Offs)Rneg=Rb*(½−((P/Pfs)*Sens)−Offs)  (1)where Rb (=Rpos+Rneg) denotes the equivalent half-bridge resistance, Pfsthe full scale pressure value, P the measured pressure, Sens the sensorsensitivity and Offs the sensor offset. The equations (1) are derivedfrom the physics of the sensor and from customer specifications. Theequations (1) are valid only if the bridge resistance (Rpos+Rneg) isindependent from the applied pressure. Otherwise, the Rpos and the Rneghave a non-linear dependence on the pressure. In most real sensors thatnon-linearity is however negligible and therefore it is omitted from thediscussions below.

When using this type of sensors, the difficulty is in finding theoptimal driving and signal read-out circuits. Ideally the circuitsshould have a differential output signal (in order to obtain a betternoise, power supply rejection ratio (PSRR) and electromagneticcompatibility (EMC) performance) and a life-time stable drivingcapability.

A differential output signal is available intrinsically if a full-bridgesensor is used. However, the use of a half-bridge circuit may bepreferred because of its lower cost and smaller physical dimensions.Hence, there is interest in finding a differential signal solutioncomprising a half-bridge circuit.

A typical application connection of such sensor includes voltage sourcedriving of the entire half-bridge as shown in FIG. 2 (node Vsupply inFIG. 2). The output is taken as a single ended node Vout. Further signalprocessing with the single ended output cannot achieve the noise, PSRRand EMC performance requirements that are typically needed.

Therefore an output differential signal is formed as shown in FIG. 3.The output differential signal is formed as the difference between thesensor single ended output node Vsensor and a portion of the supplyvoltage K*Vsupply formed by e.g. resistor division. This approach isadopted in U.S. Pat. No. 6,765,391.

However, several problems remain unsolved with this approach. First, theformation of a voltage K*Vsupply proportional to the supply voltage maybe done by resistance division (as in FIG. 3), but then a parallelbranch connected to the half-bridge supply (Vsupply in FIG. 3) isaffecting the total resistance. For many pressure sensors the equivalenthalf-bridge resistance changes over temperature and is used as input fortemperature signal processing and the resulting output signal is furtherused for correcting the pressure processing characteristics (like e.g.gain and offset) over temperature. Further, the portion K*Vsupply may beobtained through buffering prior to the resistor division. In this casethe offset and life time stability of the applied buffer limit theperformance of the sensor signal processing. Another problem is that anylife-time change of the supply voltage affects the magnitude of theoutput differential voltage, limiting the life-time performance of thesensor signal processing.

Hence, there is a need for a half-bridge circuit wherein these drawbacksare reduced or even overcome.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide for ahalf-bridge signal processing circuit wherein one or more of theabove-mentioned drawbacks are avoided or reduced.

The above objective is accomplished by the solution according to thepresent invention.

In a first aspect the invention relates to a half-bridge signalprocessing circuit comprising a first and a second branch. The firstbranch comprises a first stimulus responsive sense element and a firstcurrent source arranged to provide a current to the first sense element.The second branch comprises a second stimulus responsive sense elementand a second current source arranged to provide a current to the secondsense element, whereby the first and the second branch have a terminalin common. The first branch comprises a first node between the firstcurrent source and the first stimulus responsive sense elementconfigured to generate a first signal related to a voltage over thefirst sense element and the second branch comprises a second nodebetween the second current source and the second stimulus responsivesense element configured to generate a second signal related to avoltage over the second sense element. A differential output voltagesignal is obtained from the difference between the first and the secondsignal.

The proposed solution indeed allows for meeting the requirements as setout above. The proposed circuit yields a differential output signal,obtained via said first and second signal.

In a preferred embodiment the first and second current source areadaptive.

In embodiments of the invention the half-bridge signal processingcircuit comprises a read-out unit arranged for receiving the first andthe second signal and for reading out a common mode voltage derived fromthe first and second signal. Advantageously, the circuit then alsocomprises a feedback control unit arranged for receiving said commonmode voltage and an indication of a target range for the common modevoltage, and for producing a feedback control signal to the first andsecond current source. The feedback control signal is determined basedon a comparison of the common mode voltage and the target range.

In preferred embodiments the half-bridge signal processing circuit isarranged for determining a ratio of the differential output voltagesignal to a common mode voltage derived from the first and the secondsignal. Advantageously, the circuit comprises a calculation means tocalculate said ratio. This calculation means can be implemented inhardware or in software.

In certain embodiments the half-bridge signal processing circuit isarranged to use a common mode voltage derived from the first and thesecond signal as a reference voltage.

In another preferred embodiment the half-bridge signal processingcircuit comprises in a further branch a reference resistor and a furthercurrent source arranged to provide current to the reference resistor,whereby the further branch comprises a further node between the furthercurrent source and the reference resistor configured to output a furthersignal representing a voltage over the reference resistor.

In another embodiment the further current source is arranged to alsoreceive the feedback control signal.

In embodiments of the invention a half-bridge equivalent resistanceformed by the first and said second stimulus responsive sense elements,is temperature dependent.

In a further embodiment the half-bridge signal processing circuit isarranged to compute a ratio of the further signal (Vint) to the sensorcommon mode output voltage.

In embodiments of the invention the circuit comprises a filter forfiltering the common mode voltage, so that the impact of noise isreduced.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain objects and advantages of the invention havebeen described herein above. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

The above and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, wherein like reference numeralsrefer to like elements in the various figures.

FIG. 1 illustrates a conventional representation of a half-bridgecircuit.

FIG. 2 illustrates the typical connections of such a conventionalhalf-bridge circuit.

FIG. 3 illustrates a prior art half-bridge circuit arranged to output adifferential signal.

FIG. 4 illustrates a scheme of a half-bridge circuit according to theinvention.

FIG. 5 illustrates an embodiment with a read-out circuit and a feedbackloop.

FIG. 6 illustrates one possible way to determine the ratio Vdiff/Vcm.

FIG. 7 illustrates an alternative way to determine the ratio Vdiff/Vcm.

FIG. 8 illustrates an embodiment of the half-bridge circuit of theinvention with an additional branch.

FIG. 9 illustrates an embodiment of the half-bridge circuit of theinvention with an additional branch and feedback control block.

FIG. 10 illustrates a way to determine the ratio Vint/Vcm.

FIG. 11 illustrates another way to determine the ratio Vint/Vcm.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims.

Furthermore, the terms first, second and the like in the description andin the claims, are used for distinguishing between similar elements andnot necessarily for describing a sequence, either temporally, spatially,in ranking or in any other manner. It is to be understood that the termsso used are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

It should be noted that the use of particular terminology whendescribing certain features or aspects of the invention should not betaken to imply that the terminology is being re-defined herein to berestricted to include any specific characteristics of the features oraspects of the invention with which that terminology is associated.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

The present invention proposes a novel approach for connecting anddriving a half-bridge sensor. FIG. 4 illustrates a half-bridge circuitaccording to an embodiment of the invention. The circuit comprises twobranches which at one end receive a respective voltage supply signal(V1, V2) as a stimulus signal. At the other end both branches areconnected at terminal 10 to a common potential, which in certainembodiments can be ground potential. Each branch is provided with astimulus responsive bridge element. These elements have nominally equalstimulus sensitivities of opposite sign. The two stimulus responsivebridge elements at the other end of the first and second branch,respectively, have the terminal 10 in common to connect to that commonpotential as illustrated in FIG. 4. In the embodiment of FIG. 4 thebridge elements are resistors denoted Rpos and Rneg, respectively. Whenan identical stimulus signal is applied, the resistance of Rpos in thefirst branch increases with a certain amount, while the resistance ofthe resistor Rneg in the other path decreases over substantially thesame amount. A current source (Isupply1) is arranged to apply current tothe half-bridge circuit branch comprising Rpos. A voltage drop Vp overthe bridge element Rpos is thereby produced. Similarly, another currentsource (Isupply2) feeds a current to the branch containing Rneg, therebyproducing a voltage drop Vm over the bridge element Rneg. In this mannerthe half-bridge circuit provides intrinsically a differential outputvoltage signal (Vdiff) via the difference between the voltages Vp and Vmobtained at terminals 21 and 22, respectively.

In the embodiment shown in FIG. 5 the half-bridge circuit is arrangedfor performing sensor output common mode voltage control. A sensoroutput common mode voltage read-out circuit (15) is added to thehalf-bridge circuit. The read-out circuit takes as input the voltages Vpand Vm and outputs a read-out common mode voltage Vcm=(Vp+Vm)/2. Thiscommon-mode voltage is next fed to a feedback control unit (20). Thefeedback control unit also receives as an input a target range for thecommon mode voltage. Alternatively, the feedback control unit contains apredetermined set of suitable values for the common mode voltage. Thefeedback control unit controls if the common mode voltage falls withinthe target range. If needed, the values of the supply current sourcesare adjusted to bring the common mode voltage within its target range.

The magnitude of the physical quantity (e.g. pressure) can be determinedby processing the ratio of the sensor differential output voltage Vdiffover the sensor common mode output voltage Vcm. This can be shown withthe following equations, whereby expressions (1) are exploited

$\begin{matrix}{{Vdiff} = {{Vp} - {Vm}}} \\{= {{{{Isupply}\; 1} \star {Rpos}} - {{{Isupply}\; 2} \star {Rneg}}}} \\{= {{{Isupply}\; 1} \star \left\lbrack {{{Rb} \star \left( {{1/2} + \left( {\left( {{P/{Pfs}} \star {Sens}} \right) + {Offs}} \right)} \right\rbrack} -} \right.}} \\{{{Isupply}\; 2} \star \left\lbrack {{Rb} \star \left( {{1/2} - \left( {\left( {{P/{Pfs}} \star {Sens}} \right) - {Offs}} \right)} \right\rbrack} \right.}\end{matrix}$

Dynamic element matching techniques (such as at least two-phasechopping) can be used, which allows assuming the two supply currentsources Isupply1 and Isupply2 to be equal. Taking two-phase chopping asan example, there are two commutation phases. In phase 1 current fromIsupply1 goes through sense element 1 and current from Isupply2 goesthrough sense element 2. In phase 2 Isupply2 goes through sense element1 and Isupply1 goes through sense element 2. The samples of the twophases are then averaged. Thus, if Isupply1=I and Isupply2=I+ε thesample values in the two phases can be expressed as:Sample_Vdiff_phase1=Rpos*I−Rneg*(I+ε)Sample_Vdiff_phase2=Rpos*(I+ε)−Rneg*ISample_Vcm_phase1=Rpos*I+Rneg*(I+ε)Sample_Vcm_phase2=Rpos*(I+ε)+Rneg*IFor the ratio Pratio=Vdiff/Vcm one obtains:Pratio=(Sample_Vdiff_phase1+Sample_Vdiff_phase2)/(Sample_Vcm_phase1+Sample_Vcm_phase2)=(Rpos−Rneg)/(Rpos+Rneg)=2*[(P/Pfs)*Sens+Offs]In case another dynamic element matching technique is applied, theassumption Isupply1=Isupply2=I can still be made, leading to:Pratio=Vdiff/Vcm=(Rpos−Rneg)/(Rpos+Rneg)=2*[(P/Pfs)*Sens+Offs]

Some possible ways to determine the ratio Pratio=Vout/Vcm=Vdiff/Vcm areillustrated in FIGS. 6 and 7, respectively. In FIG. 6 the common modevoltage Vcm is determined in block 15 and processed through block 27representing an A/D converter (ADC1) with reference voltage Vref. Thedifference Vdiff between Vinp and Vinm is processed through block 29comprising another ADC with the same reference voltage Vref. In anotherembodiment it is processed through the same ADC as used for Vcm, but ina different time period. Again the same reference voltage Vref is used.Next there is in certain embodiments a digital state machine or, inother embodiments, a SW implementation to realize a digital divisionbetween the two ADC results yielding the ratio Vdiff/Vcm. In theembodiment shown in FIG. 7 a different approach is adopted: thereference voltage is now equal to Vcm and used to process Vdiff throughthe ADC. The ADC output directly yields the ratio of interest.

The sensor sensitivity and offset imperfections can be compensated forby applying a calibration. There are many ways to perform suchcalibration. In one example, four temperatures and two pressures areused. Pressure gain and offset corrections are obtained, compensatedwith third order over temperature. Temperature compensation techniquesthat can readily be applied here, are well known in the art, for examplein the papers ‘A Temperature Compensation Algorithm of PiezoresistivePressure Sensor and Software Implementation’ (D. Xu et al., Proc. IEEEConf on Mechatronics and Automation, Aug. 4-7 2013, Takamatsu, Japan)and ‘Design of temperature compensation for silicon-sapphire pressuresensor’ (H. Manguo et al., IEEE Int'l Conf on Imaging Systems andTechniques 2017, 18-20 Oct. 2017).

The signal processing required when using the proposed half-bridgecircuit is not sensitive to the imperfections and life-time changes ofthe sensor supply circuit and the sensor read-out circuit. Due to theuse of a differential sensor output signal the circuit is robust againstEMC, PSRR and noise as was desired.

In further embodiments the sensor common mode output voltage can befurther filtered, without having a big impact on the bandwidth of theprocessed signal. In this way the noise on the temperature channelduring the half-bridge equivalent resistance Rb measurement can bereduced.

In another aspect the invention proposes a solution for the processingof the half-bridge equivalent resistance Rb, which e.g. in a pressuresensor can be dependent on the temperature. The processed temperaturecan then be used for temperature compensations of the sensor sensitivityand offset. An embodiment of the corresponding half-bridge circuit isillustrated in FIG. 8. There is an additional current source Iint,matched with Isupply1 and Isupply2, which sources current over thereference resistor Rint, thereby producing a voltage drop Vint.

A feedback control mechanism can be used to control Isupply1 andIsupply2 together with the matched Iint as described above in order toobtain a relatively stable Vcm over the temperature with Vint changingover the temperature due to the Iint change forced by the feedbackcontrol. In this way a bigger magnitude is provided for the Vcm and Vintsignals, which is more suitable for further processing. FIG. 9illustrates such a scheme. If the feedback control mechanism is not used(as e.g. in FIG. 8) and all current sources are stable over thetemperature, the Vcm and Vint signal magnitude would be relatively smallfor low Rb, which in certain conditions can be an obstacle for theirfurther processing.

The ratio (Tratio) of the voltage drop on the internal referenceresistor Vint over the sensor common mode output voltage is processed.This can be described as follows:Vint=Iint*RintVcm=(Vp+Vm)/2=(Isupply1*Rpos+Isupply2*Rneg)/2Vcm=(Isupply1*[Rb*(½+(P/Pfs*Sens)+Offs)]+Isupply2*[Rb*(½−((P/Pfs*Sens)−Offs))])/2

Again, optionally dynamic element matching techniques can be applied sothat a fixed ratio can be assumed between Iint and Isupply1, Isupply2noted as Iratio. For Tratio, the ratio of the voltage drop on theinternal reference resistor Vint over the sensor common mode outputvoltage, one can writeTratio=Vint/Vcm=Tratio*Rint/(Rb/2)

The invention uses calibration for the compensation of the Tratio andRint imperfections. This signal processing is sensitive only to thelife-time changes of the Tratio and Rint.

Some possible ways to determine the ratio Tratio=Vint/Vcm areillustrated in FIGS. 10 and 11, respectively. In FIG. 10 the common modevoltage Vcm is determined in block 15 and processed through block 27representing an A/D converter (ADC1) with reference voltage Vref. Thevoltage drop over the reference resistor is processed through block 29comprising another ADC with the same reference voltage Vref. In anotherembodiment it is processed through the same ADC as used for Vcm, but ina different time period. Again the same reference voltage Vref is used.Next there is in certain embodiments a digital state machine or, inother embodiments, a software implementation to realize a digitaldivision between the two ADC results yielding the ratio Vint/Vcm. In theembodiment shown in FIG. 11 a different approach is adopted: thereference voltage is now equal to Vcm and used to process Vint throughthe ADC. The ADC output directly yields the ratio of interest.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Theforegoing description details certain embodiments of the invention. Itwill be appreciated, however, that no matter how detailed the foregoingappears in text, the invention may be practiced in many ways. Theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure and the appendedclaims. In the claims, the word “comprising” does not exclude otherelements or steps, and the indefinite article “a” or “an” does notexclude a plurality. A single processor or other unit may fulfil thefunctions of several items recited in the claims. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage. A computer program may be stored/distributed on a suitablemedium, such as an optical storage medium or a solid-state mediumsupplied together with or as part of other hardware, but may also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems. Any reference signs in the claimsshould not be construed as limiting the scope.

The invention claimed is:
 1. A signal processing circuit comprising afirst and a second branch, wherein said first branch is arranged toreceive at one end a first voltage supply signal and comprises a firststimulus responsive sense element and a first current source arranged toprovide a current to said first sense element and said second branch isarranged to receive at one end a second voltage supply signal andcomprises a second stimulus responsive sense element and a secondcurrent source arranged to provide a current to said second senseelement, whereby said first stimulus responsive sense element at theother end of said first branch and said second stimulus responsive senseelement at the other end of said second branch have a terminal in commonto connect to a common potential, and whereby said first branchcomprises a first node between said first current source and said firststimulus responsive sense element configured to generate a first signalrelated to a voltage over said first sense element, whereby said secondbranch comprises a second node between said second current source andsaid second stimulus responsive sense element configured to generate asecond signal related to a voltage over said second sense element, andwherein a differential output voltage signal is obtained from thedifference between said first and said second signal; wherein a read-outcircuit is arranged for receiving said first and said second signal andfor reading out a common mode voltage derived from said first and saidsecond signal; wherein a feedback control unit is arranged for receivingsaid common mode voltage and an indication of a target range for saidcommon mode voltage, and for producing a feedback control signal to saidfirst and said second current source.
 2. The signal processing circuitas in claim 1, wherein said first and said second current source areadaptive.
 3. The signal processing circuit as in claim 1, wherein saidfeedback control signal is determined based on a comparison of saidcommon mode voltage and said target range.
 4. The signal processingcircuit as in claim 1, arranged for determining a ratio of saiddifferential output voltage signal to said common mode voltage derivedfrom said first and said second signal.
 5. The signal processing circuitas in claim 4, comprising a calculation means to calculate said ratio.6. The signal processing circuit as in claim 1, arranged to use saidcommon mode voltage derived from said first and said second signal as areference voltage.
 7. The signal processing circuit as in claim 1,comprising in a further branch a reference resistor and a furthercurrent source arranged to provide current to said reference resistor,whereby said further branch comprises a further node between saidfurther current source and said reference resistor configured to outputa further signal representing a voltage over said reference resistor. 8.The signal processing circuit as in claim 7, wherein said furthercurrent source arranged to receive said feedback control signal.
 9. Thesignal processing circuit as in claim 8, arranged to compute a ratio ofsaid further signal to said common mode voltage.
 10. The signalprocessing circuit as in claim 1, wherein a half-bridge equivalentresistance formed by said first and said second stimulus responsivesense elements, is temperature dependent.
 11. The signal processingcircuit as in claim 1, comprising a filter for filtering said commonmode voltage.
 12. A signal processing circuit comprising a first and asecond branch, wherein said first branch is arranged to receive at oneend a first voltage supply signal and comprises a first stimulusresponsive sense element and a first current source arranged to providea current to said first sense element and said second branch is arrangedto receive at one end a second voltage supply signal and comprises asecond stimulus responsive sense element and a second current sourcearranged to provide a current to said second sense element, whereby saidfirst stimulus responsive sense element at the other end of said firstbranch and said second stimulus responsive sense element at the otherend of said second branch are directly connected to a common terminal toconnect to a common potential, and whereby said first branch comprises afirst node between said first current source and said first stimulusresponsive sense element configured to generate a first signal relatedto a voltage over said first sense element, whereby said second branchcomprises a second node between said second current source and saidsecond stimulus responsive sense element configured to generate a secondsignal related to a voltage over said second sense element, and whereina differential output voltage signal is obtained from the differencebetween said first and said second signal; wherein a read-out circuit isarranged for receiving said first and said second signal and for readingout a common mode voltage derived from said first and said secondsignal; wherein a feedback control unit is arranged for receiving saidcommon mode voltage and an indication of a target range for said commonmode voltage, and for producing a feedback control signal to said firstand said second current source.